Method and apparatus for drying a fiber web

ABSTRACT

The invention relates to a method and apparatus of drying a fiber web ( 5 ) which is dried between two tight bands ( 1, 2 ) that move in parallel and turn around turning rolls ( 6   a   , 6   b   , 7   a   , 7   b ), the first band ( 1 ) being heated and the second band ( 2 ) being cooled, the fiber web ( 5 ) being arranged to run through the drying zone defined by the bands ( 1, 2 ) together with at least two felts or wires ( 3, 4 ) such that the fiber web ( 5 ) is in contact with the surface of the first band ( 1 ), a coarse wire ( 4 ) is in contact with the surface of the second, cooled band ( 2 ), and a fine wire ( 3 ) is between the fiber web ( 5 ) and the coarse wire ( 4 ). The fine wire ( 3 ) is cleaned with a cleaning device ( 11 ).

FIELD OF THE INVENTION

The method relates to a method of drying a fiber web by an apparatuscomprising two endless bands that are impermeable to air, first turningrolls around which the first band is arranged to turn, and secondturning rolls around which the second band is arranged to turn; thefirst band and the second band being arranged to run part of the way inparallel such that they define between them a drying zone, the firstband being heated and the second band being cooled, and the fiber weband at least two felts or wires, of which at least one is a coarse wireand at least one is a fine wire being arranged to run between the bandsin such a manner that the fiber web is in contact with the first, heatedband, the coarse wire is in contact with the second, cooled band, andthe fine wire, correspondingly, is between the fiber web and the coarsewire.

The invention further relates to an apparatus for dying a fiber web, theapparatus comprising two endless bands that are impermeable to air,first turning rolls around which the first band is arranged to turn, andsecond turning rolls around which the second band is arranged to turn;the first band and the second band being arranged to run part of the wayin parallel such that they define between them a drying zone, the firstband being heated and the second band being cooled, and the fiber weband at least two felts or wires, of which at least one is a coarse wireand at least one is a fine wire being arranged to run between the bandsin such a manner that the fiber web is in contact with the first, heatedband, the coarse wire is in contact with the second, cooled band, andthe fine wire, correspondingly, is between the fiber web and the coarsewire.

BACKGROUND OF THE INVENTION

Many patent publications, such as WO 96/11300 and U.S. Pat. No.4,461,095, teach the drying of a fiber web between two parallel metalbands moving in the same direction such that the fiber web is in contactwith the heated metal band and there is a wire between the fiber web andthe second, cooled metal band, whereby steam separated from the fiberweb by heating condenses to the wire by the effect of the cold metalband. The basic idea is that two endless metal bands are arranged toturn around turning rolls and that against the inner surface of theloops formed by the bands are provided pressure chambers containing hotsteam and water, respectively, such that the pressure produced pressesthe hot and cold bands, respectively, against the fiber web and the wirerunning between them. Together with seals, the bands located between thepressure chambers define one side of the pressure chambers, whereby thesteam and the water can directly affect the bands. The operation of theapparatus is fully known per se and has been disclosed, for example, inthe above patent publications, which are incorporated herein byreference.

The basic idea of the operation of the drying apparatus is that becauseof the heat of the heated band, the water in the web is vaporized andtransferred through the wire or wires towards the cooled band. The steamproduced on the surface of the cooled band condenses into water and isremoved with the cooled band and the wire against it. U.S. Pat. No.5,594,997 teaches the removal of water from the wire against a cooledband by means of blow and/or suction boxes. In the drying apparatusesdescribed above, the fiber web is guided to and from the apparatus bymeans of a fine wire. The fine wire is placed between the coarse wirelocated against the cooled band and the fiber web, and the fine wirefurther aims to prevent the wiremark on the surface of the web caused bythe coarse wire. The small apertures of the wire against the fiber webare susceptible to being clogged, which causes the wire to adhere to thesurface of the fiber web. This, again, renders the surface of the webuneven. In addition, clogged apertures prevent the water from passingfrom the web to the coarse wire, resulting in a lowered dryingefficiency of the web.

An object of the present invention is to provide a method and anapparatus which can avoid the above disadvantages.

The method of the invention is characterized by cleaning the fine wire.

SUMMARY OF THE INVENTION

Further, the apparatus of the invention is includes a cleaning devicefor cleaning the fine wire.

The essential idea of the invention is that a fine wire is arrangedbetween the wire against the cooled band and the fiber web, and the finewire is cleaned. In a preferred embodiment, the cleaning is performed byspraying water onto the wire by a water nozzle. In another preferredembodiment, after the water nozzles, warm or hot air is blown throughthe wire from the opposite side to the water nozzles. In a thirdpreferred embodiment, after the water nozzle, warm or hot air is blownthrough the wire also from the same side of the water nozzle.

The advantage of the invention is that the small apertures of the wirecan be kept unclogged and the wire does not adhere to the surface of thewire. Further, as the apertures are unclogged, water from the web passeseasily through the wire; in all, the surface of the web can be madeeven. The water nozzles allow the cleaning to be performed extremelyefficiently; the air blowing allows harmful moisture to be dischargedfrom the wire. The moisture would also show in the moisture profile ofthe web and in the characteristics of finished paper and board. Byblowing air also from the opposite side of the wire to the water nozzlesallows moisture from the wire to be removed extremely efficiently.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described in closer detail with reference to theattached figure, which is a schematic, sectional side view of a dryingapparatus of a fiber web in accordance with the invention, taken in thetravel direction of the web.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

The figure is a schematic sectional side view of an apparatus of theinvention in the travel direction of the web. The apparatus comprises adrying device comprising a first band 1, i.e. an upper band, and asecond band 2, i.e. a lower band, that are endless, impermeable to air,have good thermal conductivity, and are preferably made of metal. A finewire or felt 3, a coarse wire 4 and a fiber web 5 pass between thosesurfaces of the bands that face each other. The fiber web 5 moves in thedirection indicated by arrow A. The fiber web is a paper web or a boardweb, for instance. The first band 1 is arranged to turn around firstturning rolls 6 a and 6 b located at the ends of the drying apparatus.Correspondingly, the second band 2 is arranged to turn around secondturning rolls 7 a, 7 b located below the first turning rolls 6 a and 6 blikewise at the ends of the drying apparatus. Wires 3 and 4 aresupported and guided by means of guide rolls 8. Since the pressureprevailing in the drying zone in the area between the bands 1 and 2 isusually different from the pressure prevailing outside or on the sidesof the bands 1 and 2, seals are arranged on both sides of the apparatusbetween or at the edges of the bands 1 and 2, said seals preventingliquid or vapour from escaping from the space between the bands 1 and 2through the sides, or vice versa. To effect the vapour heating requiredby the drying process, the drying apparatus comprises a pressure chamber9, which is located above the first band 1. The first band 1 is sealedwith seals 9 a in respect of the body 9 b of the pressure chamber 9 suchthat the steam in the pressure chamber 9 is maintained at a suitablepressure. Below the second band 2 there is a water chamber 10, whichcontains a cooling medium, such as water, that cools the second band 2.At the edges of the water chamber 10 there are seals 10 a, with whichthe second band 2 is sealed in respect of the body 10 b of the waterchamber 10.

The operation of the drying apparatus is based on heating the first band1, which comes into contact with the web 5, with hot steam contained inthe pressure chamber 9, whereby the water in the web 5 is vaporized andtransferred through the wires 3 and 4 toward the second band 2 by theeffect of the temperature of the first band 1. The second band 2, inturn, is continuously cooled with the water located below it, wherebythe steam produced on the surface thereof condenses into water and isremoved with the band 2 and the wire 4.

The fine wire 3 against the fiber web 5 is cleaned by a cleaning device11. The cleaning device 11 comprises water nozzles 12 for spraying waterfrom the side of the surface against the fiber web 5 of the fine wire 3onto the fine wire 3 and through it. After the water nozzles 12 in thetravel direction of the fine wire 3, there is a first air nozzle 13arranged on the opposite side of the fine wire 3 in the cleaning device11. Warm or hot air is blown through the fine wire 3 at a high rate bymeans of the first air nozzle 13. Further, in the travel direction ofthe fine wire 3 after the first air nozzle 13, a second air no nozzle 14is arranged on the same side of the fine wire 3 with the water nozzle12. By means of the second air nozzle, warm or hot air is also blownthrough the fine wire 3 at a high rate. The first air nozzle 13 and thesecond air nozzle 14 can also be placed in an opposite order in thetravel direction of the fine wire 3, but most preferably the order is asshown in the figure, whereby the surface of the fine wire 3 against thefiber web 5 more readily remains sufficiently dry.

A plurality of water nozzles 12 and air nozzles 13 and 14 can beprovided in the machine and/or cross direction of the fine wire 3. Thewater nozzles 12 and the air nozzles 13 and 14 can be arranged totraverse, or move back and forth, in the cross direction of the finewire 3. The nozzles are then, however, arranged with respect to eachother such that they process the same point of the fine wire 3. For thesake of clarity, traversing means 15 moving the nozzles back and forthin the cross direction of the fine wire 3 is shown as greatly simplifiedin the attached figure; for the same reason, the feeding arrangementsfor feeding air and water to the nozzles are not shown here. The airnozzles 13 and 14 can, however, be so wide that they blow air throughthe fine wire 3 along the entire width of the wire 3. The fine wire 3moves also during the cleaning in the direction indicated by arrow B, inother words the fine wire 3 is being cleaned while it is moving.

The air nozzles 13 and 14 are most preferably arranged at an angle ofapproximately 45° with respect to the weft threads and warp threads ofthe fine wire 3. The air jets can thus be made to flow more efficientlythrough the apertures between the weft threads and warp threads in thewire.

The figure and the accompanying description are only intended toillustrate the idea of the invention. The details of the invention mayvary within the scope of the claims. It is thus not essential whatpressure medium is used in the pressure chamber 9 and the water chamber10. The pressure medium in the pressure chamber 9 can thus be, forinstance, steam, air, hot fuel combustion products or water. The coolingmedium in the water chamber 10 can be, for instance, air as well aswater.

In addition to the heating by the pressure chamber 9, the first band 1can also be heated at other places in manners known per se. Further, thefirst band 1 can also be heated entirely outside the pressure chamber 9,or the fiber web 5 can be dried even without the pressure chamber 9.

Furthermore, the second band 2 can be also cooled outside the waterchamber 10 or it can also be cooled even without the water chamber 10 ina manner fully known per se.

What is claimed is:
 1. A method of drying a fiber web comprising:passing the web through an apparatus comprising first and second endlessbands that are impermeable to air, first turning rolls around which thefirst band is arranged to turn, and second turning rolls around whichthe second band is arranged to turn, portions of the first band and thesecond band being arranged to run in parallel such that said portionsdefine between them a drying zone, the first band being heated and thesecond band being cooled, and the fiber web and at least two felts orwires, of which at least one is a coarse wire and at least one is a finewire, being arranged to run between the bands in such a manner that thefiber web is in contact with the first, heated band, the coarse wire isin contact with the second, cooled band, and the fine wire is betweenthe fiber web and the coarse wire, the fine wire having warp threads andweft threads oriented perpendicularly to each other; and cleaning thefine wire with water jets and, after the fine wire is cleaned with thewater jets, drying the fine wire by blowing air with an air nozzlethrough the fine wire with the air nozzle arranged at an oblique anglewith respect to the weft threads and the warp threads of the fine wire.2. A method as claimed in claim 1, wherein water is sprayed from a firstside of the fine wire that is against the fiber web and subsequently,the fine wire is dried by blowing air from both the first side of thefine wire and an opposite second side of the fine wire.
 3. A method asclaimed in claim 2, wherein after the fine wire is cleaned with thewater jet, the fine wire is dried by blowing air from the oppositesecond side of the wire and subsequently, from the first side of thewire.
 4. A method as claimed in claim 1, wherein the fine wire iscleaned with the water nozzle traversing back and forth in a crossdirection relative to a travel direction of the fine wire.
 5. A methodas claimed in claim 1, wherein the first band is heated by a pressurechamber and the second band is cooled by a chamber comprising apressurized medium.
 6. An apparatus for drying a fiber web, theapparatus comprising: first and second endless bands that areimpermeable to air, first turning rolls around which the first band isarranged to turn, and second turning rolls around which the second bandis arranged to turn; the first band and the second band having portionsarranged to run in parallel such that said portions define between thema drying zone, the first band being heated and the second band beingcooled; at least two felts or wires, of which at least one is a coarsewire and at least one is a fine wire, arranged to run between the bandsin such a manner that the fiber web is in contact with the first, heatedband, the coarse wire is in contact with the second, cooled band, andthe fine wire, is between the fiber web and the coarse wire, the finewire having warp threads and weft threads oriented perpendicularlyrelative to each other; and a cleaning device for cleaning the finewire, the cleaning device comprising at least one water nozzle forspraying water onto the fine wire and at least one air nozzle arrangedafter the water nozzle in a travel direction of the fine wire forblowing air through the fine wire, the air nozzle being arranged at anoblique angle with respect to the weft threads and the warp threads ofthe fine wire.
 7. An apparatus as claimed in claim 6, wherein thecleaning device comprises at least first and second air nozzles arrangedafter the water nozzle in the travel direction of the fine wire andwherein the first air nozzle and the second air nozzle are arranged onopposite sides of the fine wire.
 8. An apparatus as claimed in claim 7,wherein the water nozzle is arranged on a first side of the fine wirethat is against the fiber web, after which the first air nozzle isarranged on an opposite second side of the fine wire, after which thesecond air nozzle is arranged on the first side of the fine wire.
 9. Anapparatus as claimed in claim 6, wherein the apparatus comprisestraversing means for moving the cleaning device back and forth in across direction relative to the travel direction of the fine wire. 10.An apparatus as claimed in claim 6, wherein the apparatus comprises apressure chamber arranged to heat the first band, and a chambercomprising a pressurized medium arranged to cool the second band.
 11. Amethod as claimed in claim 1, wherein the air nozzle blows air throughthe fine wire at an angle of approximately 45° relative to the warpthreads and the weft threads thereof.
 12. An apparatus as claimed inclaim 6, wherein the air nozzle is arranged to blow air through the finewire at an angle of approximately 45° relative to the warp threads andthe weft threads thereof.